In wavelength-division multiplexed systems, there is a need for filters capable of separating the different wavelength channels. As these filters must often operate at different locations, there is a further need for wavelength standards in order to avoid channel collisions and to maintain system transparency. Specifically, such filters must be capable of maintaining their preset frequencies in environments in which the temperature may change. While Fabry-Perot etalons are convenient for this purpose in lightwave systems, they have the disadvantage that they tend to be temperature sensitive. One solution to this problem is to design filters that can be preset to a desired wavelength, and then place them in a temperature-controlled environment in order to avoid temperature-dependent effects. A preferred alternative approach would be to design an etalon that is passively compensated for temperature fluctuations solely by virtue of its physical configuration. Such a filter is then completely passive, and does not require heating elements and active controls to maintain its wavelength setting in an environment in which the temperature tends to vary.
In U.S. Pat. No. 4,861,136, an etalon is disclosed which comprises a wave-guiding portion and a non-waveguiding (i.e., air gap) portion. It is an advantage of such an etalon that its frequency can be readily tuned simply by changing the length of the gap. However, as noted above, the optical length of the cavity also varies as a function of temperature, thereby detuning the filter.
Accordingly, it is the object of the present invention to provide passive temperature compensation in a Fabry-Perot etalon.